The only real reason she could give is that she thinks she met him when too young/wouldn’t choose him now and is tired of carrying the mental load (he can be a typical man at times, a bit forgetful and messy around the house). I’m told that poor SIL knows how DD is feeling and is desperate to make things work
That's a very good reason for her to want to end the marriage. They've been together 9 years. She's had 9 years of carrying the mental load and has had enough. How messy is messy? If she can't live in a mess and SIL isn't prepared to pull his weight that must be a real strain for her.
It's probably got to the stage where they've been thinking about having children and she's realized that she doesn't want to have children with HIM and wants to move on. Those types of thoughts can precipitate the end of a marriage.
I agree these are actually good reasons to end a marriage. I strongly suspect, OP, that you were given the milder version of the reasons why DD wants a divorce.
I didn't tell my parents everything, some of it was just too personal. I believe this may be the case here.
The family can all see that what DD and SIL have is very special. She is risking throwing it all away for excitement or a fairytale life which in reality doesn’t exist. I don’t think anyone can honestly say they are happy in their marriage 100% of the time.
Unless you've been living with them all the time, OP, you have no idea what their relationship is really like. The reasons your DD has so far given are major indicators that your SIL is a lazy manchild, leaving everything to your DD. Your description of him is hardly special. In fact, he sounds like the kind of manchild I tell my DD to stay away from.
A divorce now is better than your DD getting pregnant and living a miserable life with someone she doesn't love. It wouldn't be fair on any children, either.
And looking at the reality of it, I can tell you from bitter experience that having sex when you're in a relationship with someone you've fallen out of love with is just soul destroying. And that's not with all the other stuff going on, too.
Yes, this might be TMI, but OP, you are the one with rose tinted specs on here, wanting your DD to stay in an unhappy marriage for the convenience of two families. You need to understand the reality of the situation your DD is in and stop trying to put what you want onto your DD.